The present invention relates to equipment for the calibration of instruments having a temperature sensing device, said equipment comprising means to position said device in a predetermined position, means for adjusting and generating a desired temperature in said device, and means for sensing the actual temperature of said device.
The present invention further relates to a method of calibration of a temperature sensing device by exposing said device to a desired temperature, the temperature being defined and controlled by means of another temperature sensing unit.
In connection with many types of process surveying or process controlling there is a need for temperature measuring or temperature recording instruments, such as thermometers and temperature recorders, and temperature controlled instruments, such as thermostates. It is common to such instruments that a temperature is to be measured and sensed in a definite and often not very accessible place, and for this purpose a transmitting temperature sensing unit is used, mounted in a suitable holder or probe which may be inserted and located so that the sensing unit is in the desired place. The signal from the sensing unit or sensor is applied e.g. to an indicating or recording instrument, to other signal processing equipment or other equipment to enable it to cause a response according to the temperature in the defined place. For instance in power plants or in engine rooms on ships such instruments are used to a wide extent, and it may often have serious consequences, if the instrument does not operate in a reliable and accurate manner. Therefore, it is required that the instruments are calibrated precisely before being taken into use and, besides, the operational conditions may necessitate that the instruments are checked and re-adjusted at regular intervals.
For this purpose different types of portable calibration equipment have been developed, being capable of generating and maintaining various well-defined temperatures in a chamber or a pocket in which the temperature sensing unit of an instrument may easily be inserted after having been removed from its usual place of use.
From U.S. Pat. No. 3,738,174 issued to Waldron a type of calibration equipment is known based on a metal core defining a chamber or a pocket and which is provided with built-in electric heating wires. By means of controlling equipment comprising a thermosensor in the chamber, a desired temperature may be generated in the chamber, and e.g. the pointer or scale of a thermometer may then be verified and adjusted as needed, when its sensing unit is in the chamber and thus is exposed to a well-defined temperature.
Actually, such a metal core performs rather well, but still has the disadvantage that a substantial period of time is required to stabilize the chamber temperature at a desired, adjusted value, and the heat capacity of the metal core causes the calibraiton operation to be time consuming in particular when an instrument is to be calibrated in a number of succeeding scale points. When it is desired to raise the temperature, the electric resistance wire is heated and the heat dissipates subsequently to the core and through the core to the chamber and then to the sensing unit. Until the desired temperature is reached, the temperature of the heating wire must be substantially higher than the temperature ultimately desired, to dissipate heat into the core. When the desired temperature is reached the heating power is turned down to a lower value and a substantial waiting time must elapse before a stable temperature equilibrium has been reached. The temperature overshoot implies that the operative temperature range of the calibration apparatus is restricted to be well below the temperature tolerable to the wires, and that the time required to step up the temperature by a given interval becomes very long as the calibration temperature approaches the upper limit. The only way to remedy this problem is to manufacture the heating wires from special and costly high-temperature materials. The calibration operation is particularly time consuming when the core temperature is to be re-set from a higher to a lower value in which instance a cooling fan has to be used in order to accelerate a temperature decrease in the core, the fan being built into the calibration equipment. This problem is due to the rather substantial heat capacity of the core.
Core materials such as aluminium and copper are commonly used by reason of good heat conductivity, but these metals cannot withstand very high temperatures having the tendency to glow or oxidize at temperatures being higher than in the range of 500.degree.-600.degree. C. On the other hand, the known metals which may resist these high temperatures, such as stainless steel, have a considerably poorer heat conductivity and are consequently not generally suited, either.
A completely different heating concept is the so called high frequency induction heating. By this method the piece of material to be heated is placed in an alternating electromagnetic field. Provided the material in question is electrically conductive or ferromagnetic the said field will cause eddy currents or respectively alternating magnetization either effect causing heating of said piece.
U.S. Pat. No. 3,377,838 issued to Kanazawa et al. describes an apparatus for measuring transformation characteristics of metallic materials where high frequency induction is used to heat a test specimen. However, this apparatus being well suited for research purposes is not at all applicable for the calibration of temperature sensors for which it was never intended. E.g. the heat chamber is encapsulated in a vacuum vessel making the insertion and removal of a temperature probe impossible during the operation of the instrument. Futher said patent does not explain how the reference sensor should be coupled to the test specimen.